This is very interesting.
Just to make sure I am on the correct page, your intake flows like this [correct?]:
stock airbox [or AEM SRI] w/ stock MAF sensor -- stock after-MAF hose -- Prodrive turbo inlet -- 34mm restrictor -- turbo cold side -- intercooler -- intake manifold.
Just to put everything in one sequence: MAF v is normal at idle and goes up normally as throttle is opened. Except when MAF v hits 2.54, instead of continuing up as it did previously, it starts lowering until it reaches 2.2 and then it does a step jump to 2.84 where it continues on as it should. And it does this with two different MAF sensors, a stock airbox, and a AEM SRI.
When is the last time the MAF sensor operated correctly? Before the 34mm restrictor was installed?
Can you look at the signal with a O-scope or something? Maybe the signal from the MAF sensor has some electrical noise that is disrupting the normal signal at some harmonic frequency based on engine RPM [or something] and it just happens to show up at those MAF v values.
Does it only do this when you use the dyno to hold RPM at 3000 RPM?